Handbook of Exploration Geochemistry VOLUME 4 Regolith Exploration Ge Tropical and Su btropica Edited by C.R.M. BUTT Division of Exploration ,doscience CSlRO Floreat Park, WA 6014, Australia H. ZEEGERS D6partement Exploration BRGM F- 45060 Orli5ans Cedex 2, France 1992 ELSEVIER Amsterdam - London - New York Tokyo ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 211 1000 AE Amsterdam, Netherlands ISBN O 444 89095 5 Library of Congress Cataloging-in-Publication Dala Regolith exploration geochemistry in tropical and subtropical terrains / edited by C.R.M. Butt, H. Zeegers. p. cm. -- (Handbook of exploration geochemistry ; v. 4) Includes bibliographical references and indexes. ISBN 0-444-89095-5 (acid-free) , 1. Geochemical prospecting--Tropics. I. Butt, C.R.M. II. Zeegers. H. (Hubert), 1942- . III. Series. TN270.R45 1992 622 '. 13'0913--dc20 91-34317 CIP O 1992 Elsevier Science Publishers B.V. 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No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands Chapter 1.3 THE FERRUGINOUS LATERITES D. NAHON and Y. TARDY INTRODUCTION Laterites are widespread in the intertropical belt. The depth of weatherin ranges from a few metres to over 150 m depending on the age of the laterite, the regional tectonic activity, the climate, the climatic history and the nature of the parent rock. Although exhibiting a wide variety of colours, textures and petro- graphic features, laterites have markedly homogeneous mineralogical and chemi- cal compositions and do not obviously reflect the parent rocks from which they are derived. They therefore may mask the underlying geology and are a consider- able hindrance in mapping and mineral exploration. Nevertheless, many ele- ments, including Al, Fe, Mn, Co, V, P, Cr, Ni, Cu and Au, may be concentrated to ore grade in the lateritic mantle and become significant targets for exploration. The term “laterite” is commonly attributed to Buchanan (18071, who de- scribed naturally hardening surficial materials that were used as bricks (Latin: later, a brick) from Malabar, southern India. In some African dialects, red surfcial materials are called brick earth (Maignien, 1966) and laterite refers to blocks used for construction (Prescott and Pendleton, 1952). For 150 years, a considerable and controversial literature has been devoted to the definition of the term latente and two essentially different positions have emerged. Firstly, many scientists have used ‘I laterite” to designate those Fe- and Al-rich weathering products, generally formed under tropical conditions, that are either hard or become hard upon exposure to alternate wetting and drying (Pendleton, 1936; Kellogg, 1949). This definition also includes certain highly weathered material with sesquioxide-rich,humus-poor nodules, even though they may be surrounded by earthy material that does not harden (Sivarajasingham et al., 1962). Thus, from this point of view, laterites include bauxites, “ferricretes”, Fe- or Al-duricrusts, I‘ carapaces”, I‘ cuirasses”, Pisolith- or nodule-bearing forma- tions and also clay-rich (kaolinite) horizons in which concretions and mottles are present. Secondlg, Maignien (1958, 1966) and Millot (1964) have proposed that “laterite” should be extended to all weathering products that have those chemi- cal and mineralogical characteristics specific to tropical environments, rather than be restricted to those that are hard or potentially hard. In this definition, the term includes materials commonly associated with indurated ferricretes such as red or yellow ferralitic soils, tropical ferruginous soils, kaolinitic saprolites and lithomarges, all of which are soft anh cannot harden. 43 44 tures or change of volume. Petrographic studies show that primary minerals are pseudomorphically replaced by weathering products which may in turn be further fragmented-yet for a single parent grain these may exhibit parallel extinction. These observations are the basis of the concept of isovolumetric weathering (Millot and Bonifas, 1955), namely the chemical replacement of a unit volume of parent rock by an equivalent volume of the weathering product. Upwards through this horizon, the progress of weathering is expressed petro- graphically by (i) an increase in yellow to red colouration; (ii) partial then complete dissolution of the main primary minerals; (iii) decreasing induration of the rock and a marked increase in porosity; (iv) neoformation of authigenic smectites and kaolinite at the base of the horizon and, above, of predominantly well-crystallized Fe3’-bearing kaolinite associated with amorphous or poorly crystallized Fe hydroxides. Throughout the horizon, however, Fe hydroxides are essentially restricted to the sites previously occupied by Fe-bearing primary minerals. Kaolinites derived from feldspars remain white even though they contain ferric iron. Horizon 3 The fine grained saprolite is the horizon in which most primary minerals have been altered to secondary minerals such as aoliite, goethite or amorphous iron oxyhydroxides. Only quartz and resistan5F minerals (e.g. zircon, tourmaline, I chromite) remain unweathered or only partly weathered. The horizon has a considerable range in thickness, from a few metres to more than 100 m. It is a product of isovolumetric weathering, so thak the original rock fabric is perfectly preserved and recognizable. There is some redistribution of Fe hydroxides but these are diffuse and no mottles are formed. The fme saprolite is also referred to I as lithomarge, “argiles bariolées” and pallid zone. However, the latter tem can be misleading, as the horizon may exhibit a wide variety of colours, including pale green, yellow, pink, purple, red and brown as well as white. At the base, the fine saprolite has a fine grained, slightly porous texture, but it becomes massive at higher levels as the clay fraction increases. This increase is due essentially to the secondary precipitation and accumulation of authigenic kaolinite in pores that were created during the dissolution of primary minerals. This is evident petrographically by the presence of clays coating and filling microvoids. The kaolinite consists generally of small platy crystals (0.5 pm) together with minute crystals of goethite which impart an orange colour to the coatings and filings. Because the clay fraction is so abundant, internal tensions due to shrinkjng and swelling resulting from interaction with water may rework the weathering matrix. However, kaolinite is not a highly expandable clay and in general only microstructures are affected to any degree, with macrostructures such as quartz veins preserved high in the profite. This second generation of kwthite is at abmhtte accumulation within part of an isovolumetrically weath- z ~ - ‘ered horizon (Ambrosi and h’ahon, 1986);.The presence of two types of kaolinite indicates that two slightly different geochemical weathering systems are acting simultaneously to form two different facies within the one horizon, i.e. a porous and a massive facies, Kaolinite and goethite coexist in these facies as long as th’ey remain below the water-table. No ferruginous concretions are observed to form or develop in the permanently saturated (phreatic) zone (Nahon, 1976; Muller et al., 1980). Horixon 4 The mottled zone overlies the fme saprolite above the water-table, in the unsaturated (vadose) zone. Here, the pre-existing macrostructure of the parent rock is progressively destroyed. Two major types of reorganization affect this horizon. Firstly, vertical and lateral percolation of water in the unsaturated zone leads to the formation of a network of channels and tubular voids of large diameter (average 10 mm). Secondly, ferruginous spots (5-150 mm) and nodules (10-30 mm) develop, becoming both more abundant and more indurated to- wards the top of the mottled zone. This implies that iron is mobilized from areas around large (> 1 mm) pores and reprecipitated and concentrated in clay-rich areas (Tardy and Nahon, 1985). The channels and tubular voids act as receptacles for the secondary accumula- tion of kaolinite. The kaolinite occurs as coatings and fillings, and consists of minute, randomly distributed particles of detrital or authigenic origin. It has been derived from overlying horizons or upslope, either by physical translocation or dissolved in solution, and then deposited or reprecipitated lower in the profile or downslope. Because the kaolinite void fillings contain small pores, they can subsequently be ferruginized into indurated nodules. The ferruginous segregations